Regarding vacuum carburizing treatment of steels, in a treatment method called pulse carburizing, treatment is carried out by repeating alternately a carburization period in which carburizing gas is supplied into a carburizing chamber which houses therein a treatment material heated at a high temperature thereby to keep the carburizing chamber inside at a predetermined low vacuum degree (relatively higher pressure), and a diffusion period in which the carburizing gas is exhausted and carbon is diffused from the surface of the treatment material to the inside of the treatment material under a high vacuum degree (relatively lower pressure). According to this method, carburization can be carried out up to a narrow hole inside or a deep hole inside which the carburizing gas is difficult to permeate, and carburization can be carried out uniformly also in a uneven treatment material. Therefore, the pulse carburizing has been adopted as a carburizing method suitable for such the treatment material (Refer to, for example, Patent Document 1 and 2).    [Patent Document 1] JP-A-2000-1765    [Patent Document 2] JP-A-2002-194526
However, the above Patent Documents have the following problems. In a vacuum carburizing apparatus described in each of the above Patent Documents, a variable valve for pressure adjustment (including a movable orifice mechanism) is installed at a gas exhaust pipe which connects an exhaust outlet of a furnace to a vacuum exhaust device. And pressure controls in the carburizing chamber in the carburization period and in the diffusion period are performed by this variable valve which adjusts the flowing amount of the exhaust gas. Therefore, foreign matters such as soot and tar in the exhaust gas flowing into the gas exhaust pipe attach and accumulate to the variable valve, whereby poor operation of the variable valve is caused easily, and reliability and durability of the apparatus lower. Further, since the gas exhaust pipe has a large diameter, the size of the variable valve must be large, which causes increase of cost.
Further, heretofore, as described in the above Patent Document 1 (paragraph [0016]), there has been also used a carburizing apparatus in which the above variable valve is not provided but the vacuum exhaust device is connected directly to the exhaust outlet. This apparatus, generally, controls a rotation speed of a vacuum pump, but a range in which this apparatus can control pressure reaching on an intake side is narrow. Therefore, in case that a vacuum pump having a large exhaust speed is used as the vacuum exhaust device, as shown by a curve A in FIG. 4, shift (decompression) from pressure P1 in the carburization period after the carburizing gas is introduced to pressure P0 in the diffusion period is carried out quickly. However, the pressure P1 in the carburization period becomes pressure of high vacuum degree which approximates the pressure P0 in the diffusion period, so that permeation of the carburizing gas into the narrow hole inside in the carburizing period becomes insufficient. Further, in case that a vacuum pump having a small exhaust speed is used as the vacuum exhaust device, as shown by a curve B in FIG. 4, pressure P1 in the carburization period is made a desired pressure of low vacuum degree. However, after the carburizing gas is introduced, time till the pressure P1 is decreased to the pressure P0 in the diffusion period becomes long, and the number of pulses is limited, so that there are problems such as occurrence of uneven carburization, prolongation of carburization treatment time, and the like.
The invention has been made in order to solve the above conventional problems, and it is an object of the invention to provide at low cost a vacuum carburizing apparatus which can obtain a desired carburization period pressure of low vacuum degree in the pulse carburizing time, and a quick shift characteristic from carburization period pressure to diffusion period pressure, and which is excellent at reliability and durability.